Optical fibers are used for a variety of applications, especially in long-haul, high-speed optical communications systems. Optical fibers have an optical waveguide structure that acts to confine light to within a central region of the fiber. One of the many benefits of optical fibers is their ability to carry a large number of optical signals in different channels, which provides for high data transmission rates and a large bandwidth.
The increasing demand for bandwidth and higher data transmission rates has resulted in optical fibers carrying more channels and higher amounts of optical power. At some point, however, the optical power carried by the optical fiber can give rise to non-linear effects that distort the optical signals and reduce the transmission capacity of the optical communications system. Consequently, there is a practical limit to how much optical power an optical fiber can carry.
Because the optical power is confined by the waveguide structure of the optical fiber, the intensity determines the severity of non-linear effects in the optical fiber. The intensity is defined as the amount of optical power in the guided light divided by the (cross-sectional) area over which the guided light is distributed. This area is referred to in the art as the “effective area” Aeff of the optical fiber. The effective area Aeff is calculated from the electromagnetic field distribution of the light traveling within the optical fiber using techniques and methods known in the art.
It is well-known that optical fibers with large effective areas Aeff are desirable in optical transmission systems because of their relatively high power threshold for nonlinear distortion impairments. The larger the effective area Aeff, the lower the intensity and thus the less non-linear effects. Because of this feature, an optical fiber with a large effective area Aeff may be operated at higher optical powers, thereby increasing the optical signal-to-noise ratio (OSNR).
Unfortunately, the effective area Aeff of optical fibers cannot simply be increased without bound. The conventional wisdom in the art is that an effective area Aeff of about 150 μm2 is the limit for a true single-mode fiber to maintain sufficient bend robustness, (i.e., reduced loss due to bending). In some cases, an effective area Aeff of 150 μm2 may in fact already be too large for some bending-loss requirements. However, the bending loss of an optical fiber can be reduced by increasing the mode confinement and hence the cutoff wavelength of the optical fiber associated with single-mode operation. Increasing the effective area Aeff beyond present-day values would require raising the cutoff wavelength to be above the signal wavelength, thereby resulting in few-mode operation, which gives rise to undesirable optical transmission impairments such as modal dispersion and multipath interference (MPI).
Alternatives to increasing the effective area Aeff of the optical fiber to reduce adverse non-linear effects include decreasing the effective nonlinear index n2. The nonlinear physics of an optical fiber depends on the ratio n2/Aeff. However, changing n2 is difficult and the resulting effect is likely to be very small. Reducing the fiber attenuation is another alternative for better transmission performance. A lower fiber attenuation reduces the need for amplification and thus reduces the noise of the transmission link, which in turn reduces the required signal power for a given required OSNR. However, reducing the attenuation of the optical fiber impacts the optical fiber transmission system in a different way than by changing the effective area Aeff, so that these two parameters cannot be exactly traded off.
What is needed therefore is a more robust type of large-effective-area optical fiber that reduces adverse non-linear effects while also having sufficiently small bending loss.